It is now established that many human tumors are antigenic to their hosts, and that both humoral and cellular immune responses are mounted against the tumor antigens. Recent evidence suggests that circulating antitumor antibody, combined with solubilized tumor antigen, may circulate in the form of immune complexes; there is evidence that serum blocking factors, which inhibits the tumor cell killing effect of immune lymphocytes, is (at least in some cases) an immune complex. In addition, tumor patients are known to exhibit a non-specific type of immuno-suppression as shown by their poor response to skin test antigens, and decreased activity of their lymphocytes in in vitro tests of lymphocyte activity. It is proposed to study the occurrence and levels of circulating immune complexes in cancer patients, to correlate these levels with the clinical status of the patient, both as regards to tumor and the patient's immune response; it is also proposed to study the effects of these immune complexes and their components on the in vitro correlates of cell mediated immunity. In addition these complexes may provide sources of solubilized tumor antigens which could be used for studies of antigenic cross-reactions and for the preparation of reagents for radioimmunoassay of these antigens in patients. It is anticipated that the correlation of such data as: levels of circulating immune complexes, in vivo activity and in vitro correlates of cell mediated immunity, the clinical course of the patient, and the use of immunosuppressive treatments such as x-ray or chemotherapy, will enable one to arrive at a better understanding of the biology of the antitumor immune response, and will allow a more rational approach to be taken in chemotherapy and immunotherapy of human cancer.